This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. MRI Contrast agents improve the visibility of body structures by altering the relaxation times of surrounding tissue. Chemical exchange dependant saturation transfer (CEST) techniques used in combination with agents that possess a proton exchange site can be used as a contrast mechanism by significantly changing the magnitude of the water proton signal. Various chemical exchange sites have been developed as exogenous CEST contrast agents under physiological conditions. Magnetization transfer (MT) techniques using off-resonance irradiation of the macromolecular proton pool are currently utilized to generate contrast based on these macromolecule-proton interactions. Our hypothesis is that T1[unreadable] based MRI will be less susceptible to line shape changes and provide higher sensitivity in detecting CEST agents. The purpose of this subproject is to develop a sensitive and robust method to quantify the concentration of an agent exhibiting a CEST effect taken up in tissues by assessing the chemical exchange rates via spin locking. Study Aims: 1. To develop a method to isolate the presence of a CEST contrast agent and quantify the amount of agent present. 2. To validate our model using phantoms created with several CEST agents at varying pH's and concentrations. 3. To demonstrate that our model can be reliably used to quantify CEST contrast agent concentration in animal models.